Coating articles by spraying with solid particles or powders is a useful technique that can have a number of advantages over other techniques such as coating with a suspension or solution of the coating material and then drying or curing to provide a coated surface.
In spray coating techniques the particles are propelled, at a high velocity, so that they adhere firmly to a article surface and adhere to each other to form the coating layer. Generally the particles impact on the surface and deform significantly. The “splats” formed by the impacting particles build up to form a surface coating in a controlled manner allowing the thickness of the coating to be selected. Reasonable adhesion of the splats to the article can be achieved in many instances. For example with metal particles impacting on a metal article surface it is believed that dislodging of the surface oxide layer on the metal article and the metal particles results in metal (particle) to metal (article) bonding occurring.
Bonding can also be achieved between dissimilar particle and article materials. For example, for biocompatible implants, coating a generally metallic article (implant) with hydroxyapatite is an attractive proposition as it affords the prospect of providing an implant that is compatible with bone, and should more readily bind to or assimilate with the patients own bone tissue.
Current techniques used to provide hydroxyapatite (HA) coatings on articles such as titanium implants can make use of the thermal spraying technique (such as plasma spray). Other techniques such as, ion beam, magnetron and plasma sputtering processes are also known. Pulsed laser deposition has also been attempted. Thermal spraying is a line of sight process where coating powder particles are heated (at up to the melting point of the particles) and propelled at high velocity (typically 600˜800 ms−1 or even 400-1000 ms−1) on the underlying material to form a coating. The particles form a coating by mechanically interlocking together as they impact and deform on the surface being coated.
Despite the commercial attractiveness and viability associated with the use of thermal spray technology for HA coating deposition, there are important challenges, which have hindered its widespread use. The bonding between the coating and the substrate and between the splats forming the coating layer is not reliable.
The risks associated with poor coating quality are quite high. A detached coating can result in loss of functionality of the implant. Detached coating particles can cause abrasion of the joint between the implant and natural tissue or bone mineral.
Efforts to overcome the problem of poor HA coating adhesion to a metallic (titanium or titanium alloy) article have included, as summarised in U.S. Pat. No. 6,344,276, the deposition of composite coatings where a metallic layer or layers has been applied during the coating process as a pre-coat or top coat. Alternatively composite metallic/HA coatings may be applied in various ways. In U.S. Pat. No. 6,344,276 further methods for providing a hydroxapatite containing coating, which has a low dissolution rate, are described. The method involves forming a composite target of titanium (10-75% by volume) and hydroxyapatite (90-25% by volume) by cold pressing a powdered mixture of the two components. The composite target is then deposited on titanium or titanium alloy surfaces by using ion sputtering, laser ablation, or vapour deposition techniques. The resulting coating comprises an amorphous hydroxapatite/titanium layer.
Despite the various techniques described above there is still a need for further improved coating techniques especially in the demanding field of biocompatible implants.
For example rather than using a hydroxapatite-coated titanium implant in dental work it is current practice to make use of titanium implants without such coatings. The titanium dental implant is instead provided with a roughened surface. Typically such an implant is inserted into tooth/jaw and covered to protect it from damage for a period of from 2 to 6 months to allow bonding between the natural tissues/bone and the implant to occur. The implant is then used, for example as the foundation for a tooth crown. Hydroxapatite coatings should in principle allow more rapid bonding to the subject but the risk of loss of such coatings from the implant surface is generally regarded as unacceptable.
It is an object of the present invention to provide coating methods that avoid or at least alleviate one or more of the aforementioned difficulties